How to Distinguish Fillet Weld Sizes

Written by c.l. rease
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How to Distinguish Fillet Weld Sizes
(Ryan McVay/Lifesize/Getty Images)

Fillet -- pronounced fill-it -- welds provide a structural connection between two perpendicular pieces of metal. To provide the required strength, a fillet weld must be at least three quarters the thickness of the thinnest piece of metal entering the weld. Fillet weld gauges measure the width and throat size of both convex and concave welds. Using the gauges correctly ensures a structural weld with the strength required to hold the welded metal in place.

Skill level:
Moderately Easy

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Things you need

  • Wire brush
  • Rag
  • Tape measure
  • Fillet weld gauge

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  1. 1

    Clean the weld with a wire brush to expose the bare weld. Wipe the weld with a clean rag.

  2. 2

    Measure the thickness of both pieces of metal entering the fillet weld with a tape measure. Multiply the thinnest metal thickness by .75 to determine the thinnest allowable fillet weld for the weld joint. For example, a metal with a thickness of .25 multiplied by .75 would equal a minimum allowable fillet weld size of 3/16 inch.

  3. 3

    Set a fillet weld gauge for the minimum allowable weld thickness. Slide the measuring edge of the gauge along the metal until it contacts the top edge of the weld. If the gauge hits the metal above the fillet weld, the weld is smaller than the minimum measurement. If the gauge hits the weld, the weld is large but adequate for the joint.

  4. 4

    Repeat the process on the other side of the fillet weld. A fillet weld must be equal on both sides of the joint to be of adequate size.

  5. 5

    Select a convex or concave corner of the gauge relevant to the fillet size requirement. Slide the corner of the gauge on place. If the inside metal of the gauge does not touch the weld throat, the fillet weld does not meet the minimum required weld size.

Tips and warnings

  • Measure multiple points along a fillet weld to ensure a properly welded joint.

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